1. FIELD OF THE INVENTION
This invention relates to adhesives, and more particularly, epoxy adhesives.
2. DISCUSSION OF THE PRIOR ART
The automotive industry has used sheet molding compound (SMC) for manufacture of parts for automobile bodies. Sheet molding compound typically comprises specialty thermosetting polyester resins, reinforcing fibers, thickeners to increase the viscosity of the resin and to facilitate handling, catalysts to cure the resins, thermoplastic copolymers to reduce shrinkage during cure, filler to lower cost, improve the surface, or add specific properties, mold release agent, and pigment. In the manufacture of SMC, the polyester resin and low profile additives (shrink-reducing additives) are pre-blended with the catalyst, fillers, and pigment and thickening agent. This mixture (paste) is doctored onto a layer of polyethylene film, i.e. a release liner, which passes under a glass chopper where chopped glass fibers are introduced into the paste. Glass fiber length can be varied from approximately 1/2 inch up to any desired length depending upon end use requirements. In some cases continuous glass fibers are introduced with the chopped fiber for maximum reinforcing in critical areas. The glass fibers are then enveloped by another layer of paste and polyethylene film release liner. This web or sandwich is then compacted by a series of compaction rollers to ensure complete wet-out of the glass fiber reinforcement. The liner can then be removed and the resin impregnated material can be molded under heat and pressure into a desired shape.
Adhesives that have been used for the repair of parts made of SMC have been unsatisfactory. Adhesives for SMC must have the following properties:
1. Adhesion to SMC, steel, RIM urethane, RRIM urethane, RIM polyurea, reinforced nylon, and other automotive materials; PA1 2. High temperature resistance; PA1 3. Sufficient green strength to allow handling; PA1 4. Environmental resistance.
It is also desired that the adhesive demonstrate minimum read-through and read-out. Read-through is surface distortion over a bonded area that can be measured with a profilometer. Read-out is surface distortion over a bonded area not readable with a profilometer, but visually identifiable, particularly after painting or exposure to high temperature.
A major problem with sheet molding compound involves achieving a class A paint finish that is identical to painted steel body panels. This problem results from read-through of the fiberglass reinforcement, which is unacceptable for cosmetic sheet molding compound applications.
Thermoplastic copolymers have been incorporated in sheet molding compound in an effort to improve the appearance painted SMC automobile components. The use of thermoplastic copolymers in SMC formulations has resulted in severe bonding problems for conventional repair adhesives. The problem arises from the difference in surface energies between the thermosetting polyester resin and the thermoplastic copolymer additive. This difference results in phase separation, and further results in preferential orientation of the thermoplastic material at the surface of the finished SMC part. At present, there are no after market adhesives suitable for repairing parts made of SMC.
U.S. Pat. No. 4,476,285 discloses an adhesive composition for repairing damaged graphite-epoxy composite structures, particularly those of aircraft, comprising a reaction product of a bisphenol A epoxy resin and a carboxyl terminated butadiene-acrylonitrile rubber, a curing agent consisting essentially of a cycloaliphatic diamine composed of bis (p-aminecyclohexyl)methane isomers, and a catalyst consisting essentially of triphenyl phosphine.
U.S. Pat. No. 4,223,115 discloses several classes of structural adhesive systems, each of which comprises a solution or dispersion of a polymeric material in a monomer copolymerizable therewith having incorporated therein certain phosphorous-containing compounds.
U.S. Pat. No. 3,637,591 discloses an epoxide resin composition suitable for curing wherein a neutral ester of a phosphorous acid is incorporated in a mixture of an epoxide resin and a curing agent free from a BF.sub.3 -organic base complex.
U.S. Pat. No. 4,080,353 discloses titanate-phosphite adducts which serve to reduce the viscosity of filled epoxy resins, thereby permitting higher filler loading, to enhance the tinctorial power of pigments, and to render casting resins water-extendible. However, the latter two patents do not indicate that the compositions disclosed therein would be useful as adhesives for SMC.